The spectrin and actin network on the cytoplasmic side of the red cell membrane is believed to be an important determinant of cell deformability an osmotic fragility. Recent experiments done in our laboratory showed that a large molecular weight complex (approximately 27S) consisted of spectrin bands 1 & 2, band 4.1 and actin (band 5) was able to induce polymerization of muscle G-actin in the absence of salt. We plan to use both biochemical and biophysical approaches to elucidate the role of these contractile proteins in cell aging, because of the well known phenomena of decreased deformability and increased susceptability to osmotic lysis in old cells. The biochemical approach will be on the characterization of the contractile proteins in the intact cell and in isolated form, including studies on the degree of phosphorylation, state of aggregation or cross-linking, sensitivity to various agents (e.g. cytochalasins, local anesthetics, ionophores, and prostaglandins), and binding of (3H) cytochalasin B and (3H) dihydrocytochalasin B. The biophysical approach involves the measurement of the fluidity of the lipid region of the membrane, using electron paramagnetic resonance techniques, in cells of different age and under different experimental conditions. In addition, the physical properties of spectrin and actin isolated from young and old cells will be studied with viscometric and light scattering techniques. With these studies, we hope to establish a correlation between the state of the contractile proteins and the age-related rigidity of the membrane.